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As you're probably aware of but not always noticing, videos are usually shot out of order. The movie industry shoots certain scenes first, based on budget and logistics. News crews will film segments in a different order than they present them on the news. If you look carefully at movies like The Wizard of Oz, Dorothy's hair changes length several times throughout the movie. A google search of "continuity errors" will show you a raft of other goofs in TV and film resulting from this out-of-order recording.

Once the film is recorded, it's taken to be edited. "Linear editing" is generally how videos were edited before computers streamlined the process. Editing a video used to be a very time-consuming process; editors had to edit everything onto a tape sequentially, one shot after another, from the beginning to the end. If you wanted to change a series of shots in the middle of your edit, you had to reedit everything forward. For example, if you wanted to take out a scene or lengthen another scene, you had to re-edit the entire video from the changed point to the end, so there would be no gaps or lost scenes following the changed scene. If you've ever tried to combine multiple videos onto a single VHS tape, you will know what I mean.

Nonlinear editing, which is what computers are used for, give a much, much greater amount of flexibility. Programs like iMovie, Final Cut Pro, Avid Cinema, Adobe Premiere, Pinnacle Studio, and Windows Movie Maker all allow non-linear editing. A non-linear editor functions like a word processor for video; you can rearrange, insert, remove, and reformat the video without having to manually respace everything.

The way nonlinear video editors work is that they capture all the video files to a hard disk. Once all of the source video is captured and imported, you can freely assemble the video in any order you want. It can take literally hours out of video editing, and is now used from making professional Music videos to CNN segments to Hollywood films to family vacations.